Frequency multiplexers are used in order to combine a plurality of different signals into one composite signal with each of the different signals comprising a frequency component or a part of the total bandwidth of the composite signal. In order to perform the opposite operation, i.e. to separate the different signals comprised in a composite signal from a frequency multiplexer, frequency demultiplexers are used. Usually, a frequency multiplexer is reciprocal, i.e. it can be used “in the reverse direction” in order to perform demultiplexing. Likewise, frequency demultiplexers are also usually reciprocal. For this reason, although mention is mainly only made of frequency multiplexers below, the reasoning below applies to frequency demultiplexers.
It is often desired, particularly in the microwave frequency range, to have a frequency multiplexer which has as high a bandwidth as possible, i.e. a broadband frequency multiplexer, which can then be used in a number of applications, including frequency multiplexing in microwave assisted optical terabit devices and systems (Sub-Carrier Multiplexing), multi-standard and/or multi-channel communications, frequency multiplexing in high speed modems for microwave systems, ultra wideband communications and electronic warfare, in which several signals share a common antenna. Such multiplexers are also of use in test instruments, where a frequency band can be split into sub-bands in order to use a set of narrow-band function blocks
One known technique for obtaining frequency multiplexers uses closed waveguides, i.e. waveguides with a cross section which has a closed profile, usually either rectangular or elliptical. One approach within this field includes band-pass filters connected to a common junction through quarter-wave pieces of closed waveguide, and another approach is to use closed waveguide band-pass filters connected in a chain (i.e. “cascaded”) one after another, with decoupling resonators in between in order to block interaction between the band-pass filters in the chain. A third approach within this field is to use cascaded waveguide blocks comprising two hybrids and two band pass filters which successively filter out one band and sends it to one output port and sends other frequencies to another output port.
A drawback of closed waveguide frequency multiplexers is that they are expensive, bulky, and typically suitable only for narrowband applications.
Another known technique for obtaining frequency multiplexers is to use open waveguide technology in the form of microstrip lines, e.g. to make so called coupled line band-pass filters which are connected to a common junction, or to use microstrip lines to make so called combline diplexers.
A drawback of coupled line band-pass filters which are connected to a common junction is that a common junction for a number of band-pass filters makes the junction strongly frequency dependent, which means that all of the filters will interact heavily, thereby making it difficult to avoid unintentional transmission zeroes and pass-bands. In practice, the bandwidth of such open waveguide frequency multiplexers becomes limited.
A drawback of comb-line filter multiplexers is the appearance of spurious pass bands. Another drawback is that the design flexibility is limited, which means that it is difficult to obtain arbitrary filter characteristics.